I thought of that as well and it's a really great tool. However, it's probably more geared towards reproducing problems while recording really anything you do and documenting it with a screenshot. For guides or How-Tos that's maybe a bit too much and too technical. Still, I like that tool :-)
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JoeyMar 25 '10 at 19:54

i figure it can easily work in a pinch. And it's free!
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Ian BoydApr 3 '10 at 23:56

We actually develop a product, ScreenSteps, that is specifically designed to create step-by-step guides. It lets you capture screen shots and automatically assembles them into documents where you can add text, annotations, etc. It also has a sequence tool that lets you just click to add numbered circles that automatically increment in number. Makes it really easy to reference GUI elements.

Snagit is a great tool (so I gave the above answer a +1), especially for collecting a series of screenshots which you can then go back and edit (you don't have to edit each one and save it, they are just stored by the Snagit editor until you are ready, you can set the default so it does not switch to the editor every time you take a screenshot so you don't interrupt your flow.